Firefox WebDriver is provided by Selenium 2.0. To use it, you need to install Selenium 2.0 via pip:

$ [sudo] pip install selenium

It’s important to note that you also need to have Firefox and geckodriver installed in your machine and available on PATH environment variable.
Once you have it installed, there is nothing you have to do, just use it :)

An extension for firefox is a .xpi archive. To use an extension in Firefox webdriver profile you need to give the path of the extension, using the extensions keyword (passing the extensions as a list instance):